The following is excerpted from an editorial that appeared Wednesday in the Washington Post.

When Gen. David H. Petraeus and Ambassador Ryan C. Crocker last testified before Congress in September, the military results of the U.S. troop surge in Iraq, though significant, were still so preliminary that much of the debate centered on whether they were real. When the two men appeared again Tuesday, the reduction of violence had been so great as to be undeniable. Sen. Barack Obama, who predicted that the surge would not slow the bloodshed, was among the Democrats who acknowledged Tuesday that it had. Similarly, seven months ago, Petraeus and Crocker were hard-pressed to cite any movement by Iraqi leaders toward the political accords the surge was supposed to facilitate; the best Crocker could do was to say he had seen “seeds of reconciliation.” Tuesday he was able to tick off a series of significant steps, including agreement on provincial elections that could transform Iraq’s political landscape.

Petraeus and Crocker have gotten more confident about calling the surge a success, and rightly so. “It’s worth it,” said the general. “We have seen a significant degradation of al-Qaida’s presence and its abilities,” said the ambassador. …

What hasn’t much changed is the partisan debate over Iraq. … Republicans tended to follow Sen. John McCain on Tuesday in arguing that “success is within reach” and that American goals can be achieved “perhaps sooner than many imagine.” Democrats, including presidential candidates Sen. Hillary Rodham Clinton and Obama, remain locked within the “this war is lost” prism the party adopted a year ago. Tuesday Clinton, Obama and others chose to focus on the costs of the war – whether Iraq was spending too little of its own money, and whether U.S. resources would be better dedicated to Afghanistan. …

What also hasn’t changed is the sobering but firm bottom line the two envoys offer – one that neither party wants to hear. While “progress is real,” as Crocker put it, it is also “fragile” and “reversible,” as Petraeus said. That’s why Petraeus is recommending – correctly, in our view – that troop withdrawals be suspended after the five surge brigades are withdrawn and that further reductions be based on conditions in Iraq.

Contrary to McCain’s suggestion, success will require a prolonged commitment, and even then it will not be guaranteed. But the general and the ambassador both argued that such a commitment is justified. Even with all the travails of the past five years, “Iraqis, Americans and the world ultimately will judge us far more on the basis of what will happen than what has happened,” said Crocker. And an early or unconditional withdrawal would, as he noted, invite disaster “with devastating consequences for the region and the world.”